This project investigates the expression of proto-oncogenes during the differentiation of embryonic lens epithelial cells to form lens fiber cells, and seeks to determine the specific function of the corresponding gene proudcts in the developing lens. Using radioactivity labeled DNA probes we have shown that mRNA levels for two proto-oncogenes, c-myc and c-src, are specifically regulated during the differentiation of chicken embryo lens epithelial cells to form lens fiber cells, both in vivo and in vitro. Levels of c-myc mRNA are transiently evaluated during the first few hours after the initiation of differentiation in vitro, as the differentiating cells withdraw from the cell cycle. Since the c-myc gene product is a nuclear, DNA-binding protein, the transient elevation of c-myc mRNA in differentrating lens cells may lead to the regulation of other, differentiation-specific genes. Levels of c-src mRNA rise more slowly than levels of c-myc mRNA, and remain elevated in the non-dividing embryonic fiber cells. The c-src gene product is a tyrosine-specific protein kinase, the substrates of which we are presently investigating in the developing embryonic chicken lens.